Androgens mediate their effects through the androgen receptor (AR). Androgens play a role in a wide range of developmental and physiological responses and are involved in male sexual differentiation, maintenance of spermatogenesis, and male gonadotropin regulation. Several lines of evidence show that androgens are associated with the development of prostate carcinogenesis. Firstly, androgens induce prostatic carcinogenesis in rodent models and men receiving androgens in the form of anabolic steroids have a higher incidence of prostate cancer. Second, prostate cancer does not develop if humans or dogs are castrated before puberty. Castration of adult males causes involution of the prostate and apoptosis of prostatic epithelium while eliciting no effect on other male external genitalia. This dependency on androgens provides the underlying rationale for treating prostate cancer with chemical or surgical castration (androgen ablation).
Androgens also play a role in female cancers. One example is ovarian cancer where elevated levels of androgens are associated with an increased risk of developing ovarian cancer. The AR has been detected in a large majority of ovarian cancers whereas Estrogen receptor-alpha (ERa) and the progesterone receptor are detected in less than 50% of ovarian tumors.
The only effective treatment available for advanced prostate cancer is the withdrawal of androgens which are essential for the survival of prostate epithelial cells. Androgen ablation therapy causes a temporary reduction in tumor burden concomitant with a decrease in serum prostate-specific antigen (PSA). Unfortunately prostate cancer can eventually grow again in the absence of androgens (androgen-independent disease). Androgen-independent disease is biochemically characterized before the onset of symptoms by a rising titre of serum prostate-specific antigen (PSA). Once the disease becomes androgen-independent most patients succumb within two years.
The AR has distinct functional domains that include the carboxy-terminal ligand-binding domain (LBD), a DNA-binding domain (DBD) comprising two zinc finger motifs, and an N-terminus domain (NTD) that contains one or more transcriptional activation domains. Binding of androgen (ligand) to the LBD of the AR results in its activation such that the receptor can effectively bind to its specific DNA consensus site, termed the androgen response element (ARE), on the promoter and enhancer regions of “normally” androgen regulated genes (such as PSA) to initiate transcription. The AR can be activated in the absence of androgen by stimulation of the cAMP-dependent protein kinase (PKA) pathway, with interleukin-6 (IL-6) and by various growth factors. The mechanism of ligand-independent transformation of the AR has been shown to involve: 1) increased nuclear AR protein suggesting nuclear translocation; 2) increased AR/ARE complex formation; and 3) the NTD (Sadar 1999 J. Biol. Chem. 274, 7777-7783; Ueda et al 2002 A J. Biol. Chem. 277, 7076-7085; and Ueda et al 2002 B J. Biol. Chem. 277, 38087-38094). The AR may be activated in the absence of testicular androgens by alternative signal transduction pathways in androgen-independent disease, which is consistent with the finding that nuclear AR protein is present in secondary prostate cancer tumors (Kim et al 2002 Am. J. Pathol. 160, 219-226; and van der Kwast et al 1991 Inter. J. Cancer 48, 189-193).
Available inhibitors of the AR include nonsteroidal antiandrogens such as bicalutamide (Casodex™), nilutamide, and flutamide and the steroidal antiandrogen, cyproterone acetate. These antiandrogens target the LBD of the AR and predominantly fail presumably due to poor affinity and mutations that lead to activation of the AR by these same antiandrogens (Taplin, M. E., et al. Cancer Res., 59, 2511-2515 (1999)).
Conventional therapy has concentrated on androgen-dependent activation of the AR through its C-terminal domain. Recent studies developing antagonists to the AR have concentrated on the C-terminus and specifically: 1) the allosteric pocket and AF-2 activity (Estébanez-Perpiñá et al 2007, PNAS 104, 16074-16079); 2) in silico “drug repurposing” procedure for identification of nonsteroidal antagonists (Bisson et al 2007, PNAS 104, 11927-11932); and coactivator or corepressor interactions (Chang et al 2005, Mol Endocrinology 19, 2478-2490; Hur et al 2004, PLoS Biol 2, E274; Estébanez-Perpiñá et al 2005, JBC 280, 8060-8068; He et al 2004, Mol Cell 16, 425-438).
The AR-NTD is also a target for drug development (e.g. WO 2000/001813), since the NTD plays a role in activation of the AR in the absence of androgens (Sadar, M. D. 1999 J. Biol. Chem. 274, 7777-7783; Sadar M D et al 1999 Endocr Relat Cancer. 6, 487-502; Ueda et al 2002 J. Biol. Chem. 277, 7076-7085; Ueda 2002 J. Biol. Chem. 277, 38087-38094; Blaszczyk et al 2004 Clin Cancer Res. 10, 1860-9; Dehm et al 2006 J Biol Chem. 28, 27882-93; Gregory et al 2004 J Biol Chem. 279, 7119-30). The AR-NTD is important in hormonal progression of prostate cancer as shown by application of decoy molecules (Quayle et al 2007, Proc Natl Acad Sci USA. 104, 1331-1336).
While the crystal structure has been resolved for the AR carboxy-terminus domain, this has not been the case for the NTD due to its high flexibility and intrinsic disorder in solution (Reid et al 2002 J. Biol. Chem. 277, 20079-20086) thereby hampering virtual docking drug discovery approaches.